Or alternatively, why must the Olympics be held in one place at all? The infrastructural burdens of large events are massive. The World Cup distributes this burden throughout cities and even regions.
The WC is a waste as well. It's even more egregious actually, because there are more than enough existing stadiums to play the world's most popular sport. Yet FIFA still insists on new stadiums being built for just a few matches.
FIFA doesn't insist on building new stadiums so much as they award bids to countries that don't already have sufficient stadium capacity. Brazil and Qatar need/needed to construct stadiums, and so does Russia probably, but had they accepted the England and USA bids for 2018 and 2022, respectively, no stadium construction would have been required (at most, perhaps some renovation). In fact, either country could host a World Cup with only a few months' notice.
For most European countries this is not a problem. Each club which plays in its countries top division already has a big enough dedicated football stadium.
It becomes a problem when you let FIFA bribe BRIC or even third world country politicians (or vice versa). Then you get stadiums located in deserts or rainforests without a club nearby that is even remotely capable of filling it on a weekly basis.
I like what the UEFA (Europe's FIFA) is doing here: Bigger countries get to host the European Championship on their own, smaller countries share it, lately Poland + Ukraine and Switzerland + Austria. From 2020 on the Cup will not take place in a single country anymore, instead cities will apply on their own, which is of course impractical for the World Cup or even continental cups on bigger continents.
> For most European countries this is not a problem. Each club which plays in its countries top division already has a big enough dedicated football stadium.
Most in this case is maybe 5-6. Even in rich football countries like Germany, not every 1st tier club has a 40k+ stadium.
For best performance, natural grass needs to rest at least 4 or 5 days after a real game takes place. Artificial grass is not comparable, its almost a different game.